#caecilians

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A Rio Cauca caecilian (Typhlonectes natans) in Florida, USA

Last week for Wet Beast Wednesday I covered a worm that sometimes doesn't look all that wormy. This week I'm covering the opposite: a vertebrate that is trying its hardest to be a worm. Caecilians (pronounced like 'Sicilians') are obscure amphibians that don't look like amphibians at all and that's not even the weirdest thing about them.

(Image: the ringed caecilian Siphonops annulatus laying on wet leaves. It is a long, slender, limbless animal that looks like a snake or earthworm. It is a dark gray color, with narrow white rings going aorund the body all through its length. The head is similar in appearance to the tail and has two tiny eyes and nostrils. End ID)

Caecilians are a little over 220 known species of the order Gymnophiona. Gymnophiona means 'naked snake' as they were originally thought to be a group of snakes that had lost their scales. Caecilians are elongated, limbless (or in some species, having tiny, vestigial limbs) animals visually similar to earthworms and snakes. As amphibians, caecilians are vertebrates and their spines and muscles have a trait that allows them to elongate and contract their bodies. This lets them move on land or in burrows by anchoring the tail, elongating the body, and then anchoring the head and pulling the rest of the body along. When in water (or very loose mud), caecilians swim in an undulating motion like eels and some aquatic species have a fin that runs the length of the body to aid in swimming. All known species except one (Atretochoana eiselti) have lungs, usually with the left lung reduce in size compared to the right to better fit in their serpentine bodies. They have smooth skin covered with mucus and ring-like markings running down the body that look similar to the muscle bands of an earthworm. Beneath the skin is a protective layer of scales made of calcite. The skin has glands that can release toxic substances to deter predators. Caecilian skin is very thin and porous and can perform oxygen exchange. This allows them to absorb oxygen through the skin to supplement breathing, but it also renders them vulnerable to dehydration as water can escape through the skin. Caecilians need to stay wet to live and the mucus they secrete helps keep them from drying out. The largest caecilian species, Caecilia thompsoni, can reach 1.5 meters (5 ft) in length, with most species being quite a bit smaller.

(Image: the bicolored caecilian Epicrionops bicolor on a wet leaf. It has a shovel-shaped head and very narrow rings. The top of its body is purple while the underside is bright yellow. End ID)

Caecilian heads have a number of unique features. The heads are bullet-shaped and made of fused, compact bone that allows them to punch their way through soil. They have small or absent eyes and have very weak vision, only able to distinguish light from dark. The ear is simplified compared to other amphibians and in a few species, the middle ear is absent altogether. Smell appears to be their main sense as they have well-developed nostrils. In addition, Caecilians have small sensory organs called tentacles located between the nostrils. These are small barbels coated with chemoreceptors that allow them to sense chemicals in their environments. Aquatic caecilians also have the ability to sense vibrations in the water. The jaw is fused and heavier than in other amphibians. The lower jaw has two rows of teeth and some species have glands that may produce venom. The jaws have a unique double set of muscles used to close them and are more robust in burrowing species.

(Image: a close-up of a caecilian head from the side. The mouth runs down almost the entire length of the head. The eye is very small, about the same size as the nostril. A tentacle is visible as a tiny nub sticking off the tip of the nose. End ID)

Caecilians are found within South and Central America, Africa, and South and Southeast Asia. They are not known to be native to North America, Australia, or Europe, the latter of which makes me sad because it probably means there are no Sicilian caecilians. They live in warm, wet locations where the soil doesn't get dry. Most caecilians are burrowers who dig into the soil or live under leaf litter. Members of the South American family Typhlonectidae are fully or semi-aquatic. Caecilians are predators who feed on small animals like insects, worms, molluscs, and small fish. Caecilians seem to generally be non-social animals, but some are known to keep communal burrows. Because they spend most of their lives buried, very little is known about their lifestyles.

(image: an aquatic caecilian (Typhlonectes natans) in a tank. It is swimming alongside some vegetation. End ID)

Caecilians are the only amphibian order to exclusively use internal reproduction. By contrast, most frogs/toads and a few salamanders use external reproduction. Males insert an organ called the phallodeum into the female's cloaca to fertilize her eggs. Mating can last for hours. Roughly three quarters of all caecilians give live birth while the rest lay eggs. In live-birth caecilians, the larvae will develop inside the female's oviduct and she will provide food by generating unfertilized eggs that the larvae eat. Like other amphibians, offspring start out as larvae and then metamorphose into juveniles. Most live-born caecilians metamorphose internally and are born as juveniles, though some still retain external gills when born, which are shed soon after. In egg-laying species, young can hatch as larvae or as juveniles.

(Image: a Thompson's caecilian (Caecilia thompsoni) mother protecting her eggs. The mother is a long and very dark, almost black caecilian. The eggs are spherical and transparent, allowing the incubating larvae to be seen. They are smaller and lighter version of the adult and are coiled around yolk sacs. End ID)

Caecilians are unusual among amphibians for providing extensive parental care. Egg-laying mothers will often curl around their egg masses to protect them from predators. After birth many species will provide their young with food. A few species are known to engage in dermatophagy, skin eating. The mother will grow a layer of fatty skin that the young scrape off and eat with specialized teeth. These meals happen every few days (long enough for the mother to regrow the special skin layer and are fatty and nutritious enough for the young to grow 10 times their weight in a week. This behavior has been observed in very distantly related caecilians, suggesting it is more widespread than we currently realize and was present in a common ancestor as far back as the Cretaceous period. Another form of parental care is something stolen from mammals: lactation. Some species are known to suckle on a milk-like substance the mother releases from her cloaca. This substance is very fatty and allows the young to grow rapidly and the young seem to signal the mother to release her milk by making licking noise and nuzzling her. As the lactation is a relatively recent discovery, we don't know how many species do it. Caecilians may represent an evolutionary step between egg-laying and live birth similar to what the ancestors of mammals had to do (and monotremes still do). In both the cases of skin eating and lactation, it likely helps transfer the mother's microbiome to the offspring, giving them an advantage.

(Image: a mother Siphonops annulatus with her offspring. The mother is a purple-gray caecilian while the juveniles are smaller and lighter in color. The mother is coiled up in a spiral and the offspring are crawling all over her. End ID)

Because caecilians are understudied and elusive, the conservation needs of most species are unknown. They are threatened by pollution, as their thin skin can absorb toxins from the environment. The fungal disease Chytridiomycosis, which is currently in a pandemic and destroying amphibian population worldwide, may also threaten caecilians. Habitat loss through human development would also remove the fairly specific needs of most caecilian species for heavy leaf littler and constant moisture. An additional threat to caecilians in India is people. Regional folklore paints caecilians as snakes even more venomous than king cobras and they are often killed on sight. Even if caecilians are venomous, which is an open question, none are known to be dangerous to humans. An additional bit of odd caecilian folklore comes from southern Mexico and Central America, where the Mexical burrowing caecilian Dermophis mexicanus, also called the tapalcua is said to jump up the buttholes of anyone who tries to relieve themselves over their habitat. Needless to say, there is no evidence that this have ever happened (you might even call is caesilly) and no caecilian is known to be dangerous to humans. On the contrary, some are studies for potential medical applications of their slime.